1. Field of the Invention
The present invention relates to a method of operating an Atomic Force Microscope (AFM) having a probe tip attached to the free end of a lever which is fixed at the other end, and having a means to detect deflection or bending of the lever, a scanning means to scan the probe tip with respect to a sample, the detecting means being used to detect deflection or bending of the probe tip as it is scanned over the sample due to the topography, and more particularly to a method to detect the displacement of the lever due to lateral forces, which method uses the same vertical deflection detector as used for topography and produces a measure of the frictional properties of the sample surface.
2. Discussion of the Background
Atomic Force Microscopes are devices that provide three dimensional topographic images of surfaces. These devices are capable of providing resolution to atomic dimensions of surface features. In an Atomic Force Microscope (AFM), an extremely sharp tip is mounted on a small flexible lever. The tip is positioned on a surface such that the attractive forces between the surface and the tip and the repulsive force of the surface on the tip are very close to equilibrium so that the force of the tip on the surface is extremely low. If the tip is scanned laterally across a sample, the deflection of the tip will vary with the surface structure and this modulation versus lateral scan position can be used to produce an image of the surface. More typically, the sample may be servoed up and down such that the tip deflection (and thus the tip force) is kept constant during lateral scanning and the vertical adjustment signal versus lateral scan position produces a topographic map of the surface. A microscope of this type is described in U.S. Pat. No. 4,935,634, by Hansma et al. The deflection of the tip can be sensed in various ways, such as using the tunneling effect off the backside of the tip as described in a patent by Binnig, optical means such as beam deflection as described in Hansma, or interferometry. Typically, most AFMs mount the tip on a low spring-constant cantilever and sense deflection by monitoring the change in angle of reflected light off the backside of the cantilever. AFMs can operate directly on insulators as well as conductors and, therefore, can be used on materials not directly accessible to other ultra-high resolution devices such as Scanning Electron Microscopes (SEMs) or Scanning Tunneling Microscopes (STMs).
The tip in an AFM must be positioned with extreme accuracy in three dimensions relative to a sample. Motion perpendicular to the sample (z-axis) provides surface profile data. Motion parallel to the surface generates the scanning. In a typical system, the image is developed from a raster type scan, with a series of data points collected by scanning the tip along a line (x-axis), and displacing the tip perpendicularly in the image plane (y-axis); and, repeating the step and scan process until the image is complete. The precise positioning in x, y, and z is usually accomplished with a piezoelectric device. Piezoelectric devices can be made to expand or contract by applying voltages to electrodes that are placed on the piezoelectric material. The motions produced by these piezoelectric scanners can be extremely small, with some scanners having sensitivities as low as tens of angstroms per volt. The total deflection possible for these scanners is typically less than 200 microns. Scanners with different sensitivities are used for different applications, with low sensitivities used for atomic resolution images, and higher sensitivity scanners used for lower resolution, larger area images. The design of the piezoelectric scanners, including the shape of the scanner and the placement of electrodes, is well known in the art.
In an AFM, either the sample can be attached to the scanner and the tip held stationary or the tip can be attached to the scanner and the sample fixed. Typically, most existing AFMs scan the sample. This invention will describe, and the drawings will represent the case where the sample is scanned; but, the invention applies equally well to either case.
As the sample is scanned in x and y, the z axis movement is closely coupled to the tip deflection. In an AFM, either the tip deflection can be monitored as the sample is scanned, or the Z position can be varied to maintain the deflection constant with feedback. This constant deflection is called the setpoint and can be set by the control system. Modulating the z position with feedback is useful for controlling and minimizing the contact force between tip and sample, and also allows the AFM to be used for other measurements, such as stiffness.
A prior art AFM 10 is illustrated in FIG. 1, this figure being representative of the prior art AFM made by Digital Instruments, Inc., the assignee of this invention, and sold under the trademark NanoScope. The sample 12 is attached to a three-axis piezoelectric scanner 14. The sample 12 is brought into close proximity to a sharp tip 16 that is attached or part of a small, stiff cantilever 18. Some means of detecting the deflection of the cantilever is required. An optical means is illustrated, where light 20 from a small laser 22 is focused onto a reflective area on the back of the cantilever 3, and the reflected light 22' is detected by a two-element photodetector 24. The difference between outputs of the two elements of the photodetector 24 is determined, which very accurately determines the location of a spot of the reflected light 22' vertically on the photodetector 24, which is determined by the angle of the cantilever 18. As shown in FIG. 2, the laser is aligned to have the spot 26 initially centered on the dividing line between two elements of the photodetector 24 such that the difference signal is highly sensitive to bending of the cantilever 18. This is a one-dimensional detector, sensitive only to bending of the cantilever 18. The difference signal is used by the controller 28 to create the image, provide feedback to the scanner driver 30 for scanner control and other functions well known in the art. Spring constants on the order of one newton/meter and lengths of 100 microns are typical parameters for the cantilever 18. When operated in a constant deflection mode using feedback, very small forces can be applied to the sample 12.
AFMs usually are operated under control of a computer which, in some cases, controls the scanning and feedback directly; and, in most cases, acquires and displays the data. Therefore, most modern AFMs have the capability to vary scan sizes and rates, select feedback modes, and display and process entire images or individual scanlines, usually with interaction from the operator through a mouse or keyboard. Certain scan control, data display and reduction capabilities, and user interaction features are considered conventional in this application and, accordingly, are not described in detail.
The AFM described above is suitable for characterizing a surface as to properties that are functions of z for a given x, y position, such as topography and stiffness, because the deflection of the cantilever 18 due to the normal force is the measured quantity, as depicted in FIG. 3. There are, however, parameters of interest that could be measured by looking at the deflection or deformation of the cantilever due to lateral or frictional forces as depicted in FIG. 4. A useful parameter to measure at very high resolution is the surface friction. Although bulk friction can be measured, many applications, such as data storage technologies for example, use lubricants that are applied in very thin layers which can be partially worn away, and the local friction is, therefore, of interest.
Specialized local friction measuring devices using AFM techniques have been constructed by Kaneko at NTT ("A Frictional Force Microscope Controlled with an Electromagnet," Journal of Microscopy, Vol. 152, Pt. 2, 11-88) and also by Mate and McLellan at IBM ("Atomic Scale Friction of a Tungsten Tip on a Graphite Surface," Phys Rev Lett, 59, p.1942, 1987). In the IBM device, measurements were made that showed the friction due to individual graphite atoms. In both of these devices, the lateral deflection of the probe was monitored while the vertical deflection force was not precisely controlled. To measure the friction difference between a lubricated compared to an unlubricated surface, it would be more useful to measure both with a constant vertical force. Moreover, a measure of surface friction would be more meaningful if the topography data were available from the area where the friction was measured. In addition, maintaining vertical AFM capability in a friction-measuring device would allow for precise adjustment and control of the vertical force. Thus, although the above-referenced prior art has shown that it is possible to measure local friction down to atomic resolution, this capability was only demonstrated in special purpose systems without full vertical AFM capability and, in fact, was based on force-sensing devices which would be unsuitable for low force vertical AFM operation.
The AFMs described so far operate with a detection scheme that is one dimensional, either vertical or lateral. Most existing commercial AFMs just have a vertical deflection detector. One approach to achieving the dual capability of making lateral and vertical deflection measurements is to use a two-dimensional detector, such as a four-quadrant photodetector or a two-axis position-sensitive photodetector 24', as illustrated in FIG. 5. Such devices are commonly available. With appropriate electronics, which are conventional, the difference can be taken between the right and left half of the photodetector in addition to the difference between the upper and lower half. Often, as depicted in FIG. 5, a rectangular cantilever 18' is used, which will deflect vertically in response to topography; but, will also easily twist or bend to deflect laterally in response to lateral forces as the tip 16 is scanned. More common triangular shape cantilevers may also be used, as well as more unusual shapes designed specifically to maximize lateral response while maintaining good vertical response and stability. Both motions can be detected using the four-quadrant detector 24', such that the vertical deflection feedback can be maintained as the lateral deflections are measured. Such a scheme allows for equal resolution in measuring lateral deflections while maintaining the performance in measuring vertical deflections that is achieved in the prior art AFM. Several researchers have experimented with this arrangements; and, one approach using a two-axis position-sensitive detector (PSD) is described by Meyer and Amer of IBM in "Simultaneous Measurement of Lateral and Normal Forces With an Optical Beam Deflection Atomic Force Microscope" in Applied Physics Letters, 57, p.2089-2091, Nov. 12, 1990. The inventor herein has also constructed and sold an AFM of this type using a four-quadrant detector, which also measures lateral forces.
Thus, it has been shown that devices may be developed based on AFM technology that have comparable lateral force resolution to conventional AFM topographic resolution. AFMs which incorporate two-dimensional detection systems have been developed commercially. It would be desirable if possible, however, to add some type of lateral force measuring capability to existing AFMs which have one-dimensional deflection detectors. The two-dimensional detection techniques require modification of the microscope itself to incorporate additional electronics and signals, which is inconvenient for users who have already made the investment in an existing microscope; but, who wish to add friction-measuring capability.
Although the light beam deflection technique can measure light deflections in two dimensions to measure both vertical and lateral deflections, other detectors are not sensitive in two dimensions and can only measure vertical deflection. For instance, the compact AFM described in U.S. Pat. No. 5,025,658 by Elings et al, has an interferometric detector which measures cantilever height. If the light beam is placed about halfway from the fixed end to the tip end of the cantilever, then the bow of the cantilever caused by the frictional force, as shown in FIG. 4, will give a signal in the detector. This type of detector is not sensitive to the twist of the cantilever because a twist does not give a change in the average height of the cantilever.
Also, a piezoresistive strain gauge or piezoelectric strain gauge built onto or into the cantilever will be sensitive to bowing, but not to twisting. One would need two gauges per cantilever to separate bowing from twisting. The same is true for a capacitive sensor. So, being able to measure friction while being sensitive to only one dimension of motion of the cantilever is necessary for some detectors.
In addition, to achieve a qualitative measurement of lateral force differences across a sample surface, the two-dimensional technique requires a great deal of calibration. Both the lateral and vertical stiffness of the cantilevers must be measured as well as the absolute gains of the lateral and vertical measurement channels. Thus, existing frictional force AFMs make relative lateral force measurements only, while quantitative measurements would be desirable.
Friction can be a relative measure or can be defined in terms of the coefficient of friction, which is the ratio of the frictional force to the normal force. For most practical situations, the frictional force is the dominant lateral force and it is acceptable to assume that friction is the only component of the force. But, the typical AFM tip is very sharp and the normal force is low so that the contact area and loading of the tip on the surface are both small. Thus, for AFMs, often the tip is strongly affected by lateral forces that are not proportional to the normal force, such as viscous forces. For users who are interested in true frictional characteristics, the capability not found in existing instruments to separate friction from other lateral forces would be useful. Moreover, the extremely fine lateral force-measuring capability makes it possible to map other forces whose normal force distribution varies across a sample surface, such as magnetic domains or electric fields in integrated circuits, because a change in normal force will cause a detectable change in the lateral force acting on the tip, but will not change the vertical deflection if the tip is in contact with the surface. To date, mapping magnetic or electrical forces has only been accomplished with AFMs that operate with the tip not contacting the surface. These AFMs measure the effect Of the normal forces on the vertical motion of a vibrating tip.
Conventional one-dimensional detector AFMs actually do include a measurement of friction in a manner of speaking; but, this fact is not understood in the prior art. If the AFM is so arranged that the scan is along the long axis of a symmetrically-constructed scanner; then, the cantilever 18 will bend (convex or concave) toward the direction of scan as shown in the two examples of FIGS. 7a and 7b due to a frictional force F.sub.f on the tip 16. This bending will cause the light beam to move on the detector 24 just as bending due to changes in normal forces on the tip. This bending will vary with the stickiness of the surface; but, will also vary with topography. For instance, for scanning across a trench 32 with the above-described AFM arranged so that the scan is along the long axis of a symmetrically-constructed scanner, as shown in FIG. 7c, as the cantilever 18 climbs a wall 34 there will be significant bend due to a lateral force because the tip 16 will push into the wall 34 as it climbs. On the other side of the trench 32, the tip 16 will fall into the trench 32 with very little bending. This is the most extreme case, and is not actually friction related; but, shows how the tip 16 will bend due to the lateral forces, and how the direction of scan will affect the amount of bend. The key aspect of the bend is that it is similar to the bend due to topography for scans along the cantilever 18. Thus, in prior art AFMs, depending on scan direction, lateral force-induced deflections are interpreted as topography and not separately recognized for what they are. Nominally, one would expect that scanning along the cantilever axis should maximize the amount of lateral force-induced bend that is in the same direction as topography-induced bend and that scanning at 90 degrees to the axis should minimize the amount of lateral force-induced bend interpreted as topography because, at 90 degrees, the lateral force-induced bend is perpendicular to the topography-induced bend. Surprisingly, however, the inventor has discovered that in actual AFMs, asymmetries in the cantilever 18 and AFM construction can cause the angle at which the maximum lateral force-induced topography distortion occurs to be significantly different from that expected. Thus, friction is currently an uncontrolled distortion in conventional, prior art, one-dimensional AFMs.